1. Field of the Invention
The invention relates to induction heating devices for heating cooking vessels. Several such heating devices are generally combined in a domestic hotplate. Each heating device is intended to heat only one vessel at a time.
2. Description of the Prior Art
The usual kind of hotplate generally includes an induction coil fed by an inverter with current at a high frequency, for example from 20 kHz to 50 kHz. The induction coil is disposed below a support through which the magnetic field produced by the induction coil can pass. Currents are therefore induced in any metal cooking vessel placed on the support. The induced currents heat the cooking vessel, and only the vessel, which is an advantage over hot plates which apply heat by thermal conduction.
However, for maximum efficiency, the size or the diameter of the cooking vessel must be substantially the same as that of the heating or cooking area corresponding to the induction coil. If the cooking vessel is significantly larger than the heating area, induction heating is imperfect because the annular area outside the heating area is then outside the magnetic field produced by the induction coil. If the cooking vessel is significantly smaller than the heating area, the induction coil feeds current to the outside annular portion of the heating area unnecessarily because that outside annular portion does not contribute to the production of induced current to heat the cooking vessel. Once again, the heating of the cooking vessel by induction heating is therefore imperfect. The fact that in this case there is an electrical current in the annular outside portion for which there is no load, i.e. no cooking vessel, has the disadvantage that the annular outside portion generates radio frequency interference.
The combination of the heating area that can be energized by a coil and the corresponding coil is referred to hereinafter as an inductor. The problem with induction heating devices is matching the size of the energized inductor to the size of the load, i.e. the cooking vessel.
Periodic energization on a time-sharing basis of all the inductors covered by the load is known in the art. All the inductors covered by the load are energized successively during each period during the heating step. A disadvantage of this prior art is the noise caused by periodic operation of the switches for feeding the electrical current to one or other of the inductors. Another drawback of this prior art is that the load is not cooked homogeneously.
Simultaneously energizing all of the inductors covered by the load is also known in the art. Each inductor is energized by a separate generator. A disadvantage of this prior art is the cost of the heating device. Another disadvantage of this prior art is that the load is not cooked homogeneously.
One of the problems with the prior art is that the load is not cooked homogeneously. The invention proposes an induction heating device in which the particular manner of energizing the inductors during the heating step goes a long way to solving this problem. Other problems of the prior art, such as noise and cost, for example, are solved by optional additions to or preferred embodiments of the heating device according to the invention.
The invention provides an induction heating device for heating cooking vessels, the device including at least three concentric inductors and implementing a step of heating a load, wherein, for a load covering at least three inductors, at least one intermediate inductor that is covered and that is determined in such a manner as to homogenize the input of heat to the load is not energized during the heating step.
The invention will be better understood and its other features and advantages will become apparent on reading the following description and referring to the accompanying drawings, which are provided by way of example only.